1. Field of the Invention
The present invention relates to a titanium oxide photocatalyst thin film having photocatalytic activities such as antibacterial action, anti-pollution, air purification, ultra high hydrophilicity and the like on a substrate and its production method.
2. Description of the Related Art
In recent years, a photocatalyst or a photosemiconductor having the photocatalytic function or the photovoltaic function has been drawing attention. There are many application methods have been proposed, for example, titanium oxide, which is a photocatalyst (photosemiconductor), is said to oxidize and decompose organic stains adhering to surface, nitrogen oxide (NOx), sulfur oxide (SOx), air pollutants such as malodorous substances, bacteria, and the like owing to the oxidation reaction based on the photocatalytic reaction and as an practical application example, a method for removing air pollutants under sunray (Japanese Patent Application Laid-Open (JP-A) No. 6-315614) by attaching a titanium oxide photocatalyst to walls of buildings, a method for disinfecting bacteria (JP-A No. 7-102678) by attaching a titanium oxide catalyst to walls, handrails and the like in a hospital, a method for decomposing pollutants in water (JP-A No. 5-92192) by dispersing a titanium oxide catalyst powder in wastewater and radiating light of a ultraviolet lamp, a method for lessening the cleaning and maintenance work of a fluorescent lamp or luminaire (JP-A No. 9-129012) owing to self-cleaning reaction, and the like.
Further, based on its photoreaction, a photocatalytic thin film is known to have surface made highly hydrophilic and a variety of applications for anticlouding of mirrors (of bathrooms, automobiles), lens, glass windows and the like are supposed to be possible.
Further, when the photocatalytic thin film is disposed on the surfaces of building outer walls, automotive glass, and window glass, based on the hydrophilicity of the film surfaces, besides that hydrophobic stains are hard to adhere to, even if stains adhere, they are decomposed and owing to the hydrophilicity of the forgoing photocatalytic thin film, the stains or their decomposed substances are known to have self-cleaning function that they are easily washed out by rain or washing with water.
With respect to the foregoing photocatalytic thin film, the following methods are generally well known: a titanium compound such as a titanium alkoxide, a titanium acetate and the like are hydrolyzed and then applied to the surface of a substrate and dried and after that, sintered at 500° C. or higher to obtain an anatase type titanium oxide film; after an amorphous titanium oxide layer is formed by a deposition method, the obtained amorphous titanium oxide layer is annealed at 400° C. or higher to form an anatase type titanium oxide-containing layer; the surface of metal titanium is oxidized at 500° C. or higher to crystallized the surface; while a substrate being heated at 250° C. or higher, an anatase type titanium oxide film is obtained by RF sputtering method.
The foregoing photocatalytic thin film is provided with its hydrophilicity upon receiving ultraviolet rays. However, the hydrophilicity has a characteristic in that the hydrophilicity is weakened if it is kept in a dark place for about a weak. In order to improve the defects, JP-A No. 2001-98187 discloses a photocatalytic hydrophilic member bearing on the substrate surface, a surface layer including a photocatalytic titanium oxide coated with an alkali silicate and an inorganic acid particle (silica or the like) with an isoelectric point of pH 5 or lower and when an alkali silicate other than the photocatalyst is contained in the surface layer, the surface is provided with hydrophilicity as high degree as 20° or lower water wettability angle and the hydrophilicity retention property in the case of storage in a dark place is improved. Further, JP-A No. 9-57912 discloses a composite material having good water hydrophilicity retention property for a long duration even in the case of storage in the dark by forming a hydrophilicity layer such as silica or silicone resin (containing hydroxy group) on a photocatalytic semiconductor thin film.
All these methods are either a titanium oxide particle or a titanium oxide thin film coated with silica or the like on the surface and owing to the surface coating film existence, they have a problem that the photocatalytic capability such as an organic decomposition activity by the titanium oxide particle or the titanium oxide thin film is deteriorated. Further, in order to increase the photocatalytic activity, a method for solving the problem is that the photocatalytic film is thickened, however the method has a disadvantage that interference coloration occurs since titanium oxide has a high refractive index and therefore, the thickness cannot be thickened so much.